Well, that's right stigweard. It's again a question of sensitivity and thoughtfulness. I myself have joined in a session, and can usually play along with most tunes, but occasionally come across one that I can't follow very well - in which case I'll drop out. Others don't seem to think it matters if they play the wrong chords or whatever.

The other thing that annoys me when people play along with me if I'm meant to be doing a solo turn, is that I do my own arrangements of songs that don't always use the 'normal' chords, so if others join in without being asked, it can clash with what I'm doing, rather than improve it. I had this type of experience recently and I do find it off-putting. But I was too polite or timid to ask the guy to stop (he was a regular at the session and I was a visitor, so I bit my tongue).